The biochemical role of melatonin has been described already in Chem Eng News, May 1, 1967, p. 40.
Melatonin is one of the principal hormones secreted by the pineal gland in all vertebrates. This gland expresses an overt rhythm of melatonin production which results in the nocturnal synthesis and secretion of melatonin into the blood. The melatonin rhythm provides a signal reflecting the changing environmental lighting cycle in the organism. A large body of evidence indicates that melatonin is involved in the coordination of timed physiological processes. This is evident in seasonal breeders such as hamsters and sheep; the photoperiodic regulation of reproduction is manifested as a potent influence of melatonin on gonadal activity. Tomarkin et al, Science 227, 714 (1985). It has been shown (Zisapel et al, Res. Comm. 104 (1982) and Brain Research 246 161 (1983) that melatonin inhibits the stimulated release of dopamine from rat hypothalamus.
This provides means for the in vitro testing of putative melatonin antagonists.